A Dual Isotope Conundrum: Unraveling a Cryptic Subsurface N Cycle

氮是一种重要的常量营养素，在很大程度上调节着全球许多生态系统的初级生产力。通过人类活动，全球氮的周转量增加了一倍以上，导致基本生态系统动态的大规模和广泛的改变，对人类和生态健康产生显著影响。因此，准确了解氮的（生物）地球化学循环对于预测其在环境中的命运至关重要。该项目旨在通过全面研究NO3-（和NO2-）和Fe之间的生物和非生物相互作用及其对双重同位素组成的影响（例如，δ 15N和δ 18O）。具体而言，存在一个根本的差异耦合N和O同位素效应之间的微生物培养为基础的同位素研究的反硝化作用和现场为基础的同位素测量地下水NO3-在厌氧条件下的损失（反硝化作用被广泛牵连）。我们认为，这种差异是一个普遍存在的，但在很大程度上未被认识到，耦合非生物和/或生物铁循环与还原和厌氧氧化N循环途径的结果，并有初步的数据支持这一假设。这一假设的检验将有助于揭示这一明显的同位素难题的根本原因，并将有重要的意义，提高我们的理解耦合的地下N，C和Fe循环在陆地地下。该项目还将支持在马萨诸塞州举办为期两天的年度教师培训讲习班，以支持科学、技术、工程和数学教育。我们将与瓦奎特湾国家河口研究保护区（WBNERR）合作，举办一个研讨会（约15名高中教师），重点讨论流域废水问题和沿海生态系统中的氮循环。研讨会将使与会者了解自然氮循环、人类影响和环境效应，以及尽量减少人为氮负荷对当地流域和脆弱沿海生态系统影响的方法。

Nitrogen is an essential macronutrient that largely regulates primary productivity across a host of global ecosystems. Through human activities, global turnover of N has more than doubled, resulting in massive and widespread alteration of fundamental ecosystem dynamics leading to pronounced impacts on both human and ecological health. An accurate understanding of the (bio)geochemical cycling of N is, therefore, of central importance to enabling the prediction of its fate in the environment. This project aims to quantify linkages between the N and Fe cycles in the subsurface terrestrial biosphere, through a comprehensive examination of the biotic and abiotic interactions between NO3- (and NO2-) and Fe and their influence on the dual isotopic composition (e.g., delta 15N and delta 18O) of NO3- in the environment. Specifically, there exists a fundamental discrepancy of coupled N and O isotope effects between microbial culture-based isotopic studies of denitrification and field-based isotopic measurements of groundwater NO3- loss under anaerobic conditions (in which denitrification is widely implicated). We suggest that this discrepancy is the result of a ubiquitous, yet largely unrecognized, coupling of abiotic and/or biotic Fe cycling with both reductive and anaerobic oxidative N cycling pathways and have preliminary data supporting this hypothesis. Testing of this hypothesis will help reveal the underlying causes of this apparent isotope conundrum and will have important implications for improving our understanding of the coupling of subsurface N, C and Fe cycling in the terrestrial subsurface. This project will also support the development of an annual 2-day teacher-training workshop in support of Science, Technology, Engineering and Math education in Massachusetts. We will partner with the Waquoit Bay National Estuarine Research Reserve (WBNERR) to conduct a workshop (about 15 high school teachers) focused on wastewater issues in watersheds and nitrogen cycling in coastal ecosystems. The workshop will provide participants with an understanding of the natural nitrogen cycle, human impacts and environmental effects and approaches for minimizing the impact of anthropogenic N loading in local watersheds and fragile coastal ecosystems.